1. Field of the Invention
The present invention relates generally to mixing devices, and more particularly to a mixing device suited for a microfluidic environment.
2. Description of the Prior Art
Mixing fluids efficiently when the mixing volume is both temporally and spatially small poses problems. If the nature of the fluid changes upon mixing, for example, in terms of viscosity, flow dynamics are altered and the mixing efficiency is further lowered. An increase in viscosity delays the transition to turbulence, leading to lower mixing efficiencies, as the only mixing may occur by diffusion at the boundaries between the fluids.
There are several instances when it is desirable to achieve maximal mixing in a very short duration. Sub-optimal interaction of the fluids to be mixed leads to no or incomplete reaction. Mixing optimization in a microfluidic environment poses more problems because the volumes of the fluids involved are too small to use large conventional mixers. In addition to the chemical field, which involves small reactant volumes, the rapidly growing fields of drug discovery and modern biotechnology in general often encounter situations wherein bioefficacy testing or the effect of micro-volumes of molecules on cells, particles or other bioactive reagents have to be accurately studied. The difficulty of isolation and the cost of synthesis preclude testing of large volumes of compounds. Thus it becomes necessary to ensure that very small amounts of compounds are able to interact optimally so as to render accurate results.
Micromixing will be valuable for any application in biotechnology where small fluid volumes need to be mixed. In a confluence of two or more fluids at low volume, for example less than 1 microliter, and in dimensions of 100 micrometers, mixing primarily takes place by diffusion at their common boundaries. Consequently, mixing is very poor if the duration of the interaction is short. Also, if the flow is laminar, efficiency of mixing becomes even poorer (Beard, D. A., Taylor dispersion of a solute in a microfluidic channel, J. Applied Physics, 89: 4667-4669, 2001; Brody et al., Biotechnology at low Reynolds numbers, Biophys. J., 71:3430-3441, 1996; Knight et al., Hydrodynamic focusing on a silicon chip: mixing nanoliters in microseconds, Phys. Rev. Lett. 80:3863-3866, 1998, the entire contents and disclosures of which are hereby incorporated by reference herein). It is well known that when viscosity of a fluid increases, diffusion decreases, contributing to poor mixing. Thus in situations where cells or particulate matter are added to a free-flowing fluid medium as in many bioanalytical systems, interactions of the constituents may be sub-optimal. In the micron size range, small Reynolds numbers govern the delivery of aqueous samples. As fluid transport systems get progressively smaller, viscous forces dominate over inertial forces, thus rendering turbulence nonexistent. This problem is acute in microfluidics (Ethers et al., Mixing in the offstream of a microchannel system, Chemical Engineering and Processing, 39:291-298, 2001). There are times when the reaction must take place in a sterile or aseptic environment without extraneous contaminants. At other times, the reaction may peak soon after the reactants come into contact and a read-out may not be possible or may become inaccurate, if delayed. Where the design limitations stipulate for sterility, a short mixing interval and a laminar-flow, in-flow mechanisms for bringing about effective mixing within the tube or channel become desirable and sometimes critical to effective means of measurement and analysis. Commonly used microfluidic mixers are diffusion-enhanced or highly complex and require a few seconds to achieve thorough mixing: They have not been able to address most of the above limitations effectively. Thus the need for in-flow mixing mechanisms for bringing about optimal mixing of a plurality of microfluids is still unmet.